A trajectory planner is designed to work with the pulse delivery system available, if a designer only had the option of sequentially stepped axes that would influence the design of the trajectory planner rather than making generation of the required path impossible, at certain speeds there will be points on a given curve where synchronisation of pulses will not be optimal for generating the path and points where it will, the trajectory planner simply delivers the least worst pulse pattern possible.
From the point of view of generating smooth curved paths neither synchronised nor serial pulses would be an optimal method under all circumstances, in the real world with the pulse rates and given the inertia of the motors and machine it probably becomes irrelevant unless working with a perfectly rigid machine on detail which comes very close to the resolution capability of the machine,

- Nick